1. Field of the Invention
The present invention relates generally to a backlight unit that is used as the light emitting source of a display device, such as a liquid crystal display, and, more particularly, to a backlight unit that uses light emitting diodes as a light emitting source.
2. Description of the Related Art
With the development of the electronic device industry, not only a variety of small-sized and low-energy-consumption display devices but also imaging apparatuses, computers and mobile communication terminals using the display devices are being developed. Liquid Crystal Displays (LCDs), which were developed to meet such a trend, are being popularized as display devices for monitors and mobile communication terminals.
Such an LCD is generally provided with a backlight unit, which is composed of a light source for generating light at the back or side of an LCD panel and a light guide plate, because the LCD cannot intrinsically generate light. In this case, the backlight unit enables the colors of images, which are realized by the LCD panel, to be accurately reproduced by generating white color light.
In the conventional backlight unit of the LCD, a Cold Cathode. Fluorescent Lamp (CCFL) or External Electrode Fluorescent Lamp (EEFL) has been employed as a light source. However, the conventional light sources, such as the CCFL and the EEFL, have the following disadvantages. That is, most of the CCFLs and EEFLs employ the plasma principle, so that the life spans thereof are shortened because there is variation in the pressure of plasma gas, and an inverter is required to obtain an operating voltage of up to several hundred volts that is necessary for the discharge of plasma. Meanwhile, in the case where such a backlight unit is applied to portable products, such as a mobile communication terminal, most of the power is consumed by the backlight unit. The CCFL and the EEFL are disadvantageous in that they have poor power consumption efficiencies and, therefore, consume excessive power.
In order to overcome the above-described disadvantages, a backlight unit using Light Emitting Diodes (LEDs) was proposed. An LED is a luminous device that generates light using a luminescence phenomenon (referred to as “electroluminescence”) that occurs when voltage is applied to a semiconductor. Materials, which satisfy the conditions that light emission wavelengths exist in a visual or near infrared light region, light emission efficiencies are high and the formation of a p-n junction can be achieved, are suitable materials for the LED. Such materials include compounds, such as gallium nitride GaN, gallium arsenide GaAs, gallium phosphide GaP, gallium-arsenic-phosphorous GaAs1-xPx, gallium-aluminum-phosphorous Ga1-xAlxAs, indium phosphide InP, and indium-gallium-phosphorous In1-xGaxP.
Light emission may be classified into that generated by the recombination of free carriers and that generated by recombination at the luminous center of impurities. In this case, the wavelength of light generated by the recombination at the luminous center of impurities varies with the kind of impurities that are added to a semiconductor. For example, in the case of gallium phosphide, the light emission in which zinc and oxide atoms are involved is red (wavelength: 700 nm), and the light emission in which nitrogen atoms are involved is green (wavelength: 500 nm). That is, light generated by the LED has a specific characteristic color (wavelength) according to the kind of impurities and the material of a semiconductor. The LED is advantageous in that it has a small size and a long life span, and has high energy efficiency and can be operated at a low operating voltage because electrical energy is directly converted into light energy, compared to the conventional light sources. Accordingly, the LED having such advantages is being widely used as the light source of a backlight unit for an LCD.
FIG. 1 is a diagram showing the structure of a conventional LED package for a backlight unit.
Referring to FIG. 1, the conventional LED package 10 for a backlight unit includes a body 11 provided with a central cavity, two electrode plates 12 and 13 formed in the central cavity, and an LED chip 14.
The cavity is formed in the body 11 to allow the LED chip 14 to be mounted therein, and is filled with transparent resin. The sidewall of the cavity may be formed to be inclined at a predetermined angle and, therefore, guide light.
The electrode plates 12 and 13 are thin plates that are formed in the cavity with the first ends thereof being exposed to the cavity and the second ends thereof being implanted into the body 11.
The LED chip 14 may be mounted on one of the electrode plates 12 and 13. In this case, the anode and cathode terminals of the LED chip 14 are connected to the electrode plates 12 and 13, respectively, and further connected to an external circuit through the electrode plates 12 and 13, so that the LED chip 14 can be operated. For example, as shown in FIG. 1, the LED chip 14 is mounted on the first electrode plate 12, and the anode and cathode terminals of the LED chip 14 are wire-bonded to the first electrode plate 12 and the second electrode plate 13, respectively, thus being connected to the external circuit.
FIG. 2 is a diagram showing a structure in which the conventional LED package for a backlight unit is applied to an LCD, and the effects of the structure.
FIG. 2 illustrates the case where a plurality of LED packages 10 is arranged on one side of an LCD 20 as an example. Referring to FIG. 2, a conventional LED package 10 for a backlight unit has a structure in which a cavity is formed in a body 10 and an LED chip 14 is mounted in the cavity. Accordingly, in the case where the LED packages 10 are arranged on one side of the LCD 20, light is directly incident on the ranges 21 of the LCD 20 defined by the extension lines of the inclined sidewalls of the cavities of the LED packages 10 so that the ranges 21 become bright. In contrast, shadows are formed on the ranges on which light is not directly incident. As a result, the case where the conventional LED packages 10 are used as a backlight is disadvantageous in that light is nonuniformly incident on the LCD 20, so that a stain phenomenon occurs on the LCD 20, thus degrading image quality.